1. Field of the Invention
This invention relates to an apparatus for improving the performance of a TEM test cell, and more particularly to an improvement in TEM mode test cells to achieve optimum bandwidth of the cell.
2. Brief Description of the Prior Art
TEM mode test chambers or cells are known in the art. Such cells consist of a section of rectangular transmission line, operating in the transverse electromagnetic mode (TEM). The cell has tapered ends leading to a transition section that includes standard coaxial connectors defining input and output ports for the cell. Although typically a TEM cell has input and output ports, it may be useful, with proper internal loading, to provide only one connector port. The object of the design of TEM test chambers is to create a uniform electromagnetic field within the cell so as to effect measurements within .+-.1 dB, while maintaining 50 ohm impedance at both ends of the cell.
Each cell is equipped with at least one hinged access door that provides clearance for objects up to the maximum size appropriate for each cell. Additional access openings may be provided for installation of test probes, connectors, and the like.
As background material for the type of TEM test cell to which the present invention pertains, reference is made to a U.S. Department of Commerce, National Bureau of Standards publication, NBS Technical Note 1013 entitled "Using a TEM Cell for EMC Measurements of Electronic Equipment", published April 1979 and authored by M. L. Crawford and J. L. Workman. The technical note may be ordered from the Superintendent of Documents, U.S. Government Printing Office, Washington, D.C., 20402, SD Stock No. SN003-003-02053-2, such publication incorporated herein by reference.
In the aforementioned NBS publication, procedures are given for using a TEM cell for performing either standard radiated electromagnetic (EM) susceptibility testing or for measuring radiated EM emissions from electronic/electromechanical equipment. The present invention is useful for both types of operation.
TEM cells have shown great potential for performing electromagnetic interference/electromagnetic compatibility (EMI/EMC) measurements with ease, versatility, and accuracy. The TEM cell consists of a section of rectangular, coaxially line that serves as a broadband, linear phase and amplitude transducer (in the sense that it converts field strength to rf voltage or rf voltage to field strength) either to establish standard EM fields for susceptibility testing of electronic equipment or for detecting radiated emanations from electronic equipment. The cell is bounded by a shielded metallic enclosure or housing, thus providing electrical isolation for the tests being performed.
In the center of a TEM cell is a conductor, formed of a flat sheet of metal supported by dielectric rods in the center of the cell. Access to the cell is by hinged doors located at the sides of the cell.
The center conductor, referred to in the field as a septum, is required to have a specific geometry and spacing from the shielded housing enclosure in order to generate the TEM wave in the center of the cell. In order to place the objects under evaluation inside the cell, it is desirable to have the center section of the septum and shielded housing as large as possible.
It has been found that in order to generate a reasonably uniform field at the center of the septum, the minimum lateral width of the septum and the dimensions of the shielded enclosure are such that the characteristic impedance at the center of the septum must be maintained at 50 ohms. Since the electrical connectors attached to the ports of the cell typically have a characteristic impedance of 50 ohms, it is necessary to employ some means of maintaining the 50 ohm impedance along the entire length of the cell.
In the past, the method of achieving a low VSWR has been accomplished by providing a triangular shaped or linearly tapered end sections of the septum, narrowing toward the connector ports. Since the shielded housing enclosure also tapers toward the connector ports, it will be appreciated that maintaining a perfectly matched 50 ohm characteristic impedance during the transition is difficult. In order to make the smoothest transition between the 50 ohm cables attached to the connector ports and the septum, the prior art has also provided a lumped termination network having equivalent inductance and capacitance characteristics at the high frequencies of concern to make the impedance match at the septum/connector transition, thereby defining a termination network at the ports of the cell.
Examples of such prior art technology are the IFI Crawford Cell TEM Test Chambers, models CC-103 and BC-110 manufactured by Instruments For Industry, Inc., 731 Union Parkway, Ronkonkoma, N.Y., 11779.
As further background information, a TEM mode wave is characterized by orthogonal electric and magnetic fields which are perpendicular to the direction of propagation along the length of the cell or transmission line. The electric and magnetic field components are very uniform over approximately one third the volume between the septum and the outer wall of the cell. The field simulates the planar field generated in free space with an impedance of approximately 50 ohms.
The TEM mode has no low frequency cutoff. This allows the cell to be used at frequencies as low as desired. The TEM mode has linear phase and constant amplitude response as a function of frequency. This makes it possible to use the cell to generate or detect a known field intensity. The upper useful frequency for a cell, however, is limited by distortion of the test signal caused by resonances and multi-moding that occur within the cell. These effects are a function of the physical size of the cell and its components.
The electrical characteristics of a cell as a transmission line can be determined by measuring the voltage standing wave ratio (VSWR) at the cell's measurement ports, and measuring its insertion loss.
The field generated inside the cell can be calculated by: EQU E=V/B
Where:
E=field generated in volts/meter PA1 B=separation between the center plate (septum) and the outer wall PA1 V=voltage measured at the output port.
The TEM field can be produced with an accuracy of .+-.1 dB up to a frequency where the higher order modes are generated within the cell. The first resonance is demonstrated by a high VSWR over a narrow frequency range. The high Q of the cell is responsible for such high VSWR.
Insertion loss is measured by comparing the ratio of the power at the output port compared to the power at the input port of the cell.
As previously noted, the normal technique for impedance matching to lower the cells VSWR is to introduce conjugate impedance matching elements at the input and output ports of the cell. While effective, these techniques can create higher order modes in the near vicinity of the input and output ports which limit the bandwidth of the cell. While prior art devices, making use of a linear taper of the ends of the septum, have successfully reduced the VSWR within the cell, the lumped impedance matching elements at the ports create extreme distortions in field structure, resulting in a less uniform field where it is desired, i.e. at the center of the cell.
There is therefore a need in the art for an improved impedance matching arrangement which accomplishes the objective of lowering the VSWR while not having an adverse effect on the electromagnetic field in the center of a cell. The present invention accomplishes that goal.